1. Field of the Invention
This invention relates to high pressure hydraulic systems and, more particularly, to methods and apparatus for providing a flow of high pressure hydraulic fluid to a utilization device.
2. Description of the Prior Art
In the art of providing hydraulic fluid at high pressure, for use in hydrostatic forming systems or other utilization devices, it is known to utilize a single-acting pressure intensifier to increase the pressure of the fluid to a relatively high level. Typically, such a single-acting pressure intensifier includes a compound piston or ram, having a relatively large area on one face and a relatively small area on an opposite face, the compound piston being housed within a similarly configured, compound cylinder. A relatively low pressure applied to the larger face of the piston in the larger section of the compound cylinder causes a relatively high pressure to be produced in the hydraulic fluid at the smaller face of the piston in the smaller section of the compound cylinder during a pressurizing stroke of the compound piston. Such a pressure intensifier may be designated as single-acting in that it provides a pressurizing stroke in only one direction during linear reciprocation.
Single-acting pressure intensifiers of the type described are useful devices for delivering relatively short spurts of hydraulic fluid at high pressure, but are capable only of intermittently active operation. During an active phase of the operation of such a pressure intensifier, i.e., in the course of its pressurizing stroke, the required high pressure hydraulic fluid is delivered to the utilization device. However, between successive active phases of operation there must always be present inactive phases, corresponding to return strokes of the piston, such that a single pressure intensifier cannot deliver the high pressure hydraulic fluid to the utilization device continuously over extended periods of time.
In order to avoid the discontinuity of flow caused by the intermittent activity capability of individual single-acting hydraulic pressure intensifiers, resort has been made, in the prior art, to a technique of coupling two single-acting pressure intensifiers in parallel between a source of hydraulic fluid at relatively low pressure and a utilization device, and operating the intensifiers alternatingly. Such technique is exemplified by U.S. Pat. No. 527,981 to C. P. Higgins. In theory, at any one time, one of the intensifiers is in its active phase of operation, delivering hydraulic fluid at relatively high pressure to the utilization device, while the other intensifier is in its inactive phase. Conceptually, this technique would appear to have overcome the problem of discontinuous availability of high pressure fluid. However, it has been found, in practice, that the magnitude of the output pressure is not uniform throughout the pressurizing stroke of the conventional pressure intensifier. Instead, only a relatively low level of pressure intensification can be achieved when the piston is just commencing its pressurizing stroke and when it is approaching, and then attaining, the end of such stroke. As a result, periods of relatively low pressure, and variations in pressure and in flow, are characteristic of such systems for operating two, parallel-coupled, single-acting pressure intensifiers alternatingly.
Another technique which attempts to provide a continuous flow of high pressure hydraulic fluid is taught in U.S. Pat. No. 2,508,298 to O. J. Saari. This technique uses two double-acting pressure intensifiers, i.e., pressure intensifiers which are so structured as to provide pressurizing strokes in both directions of linear reciprocation. The two pressure intensifiers are coupled in parallel between a pressure source and an output path to a utilization device. Two control valves are used, each associated with a different one of the two double-acting pressure intensifiers, and each controlling the direction of movement of the compound piston of its respective intensifier. A reversal of the condition of the control valve for one of the compound pistons, causing a reversal in the direction of movement of such compound piston, is directly triggered upon the other compound piston, which constitutes, in effect, a control piston, attaining a triggering position in which it opens a conduit leading to such control valve. Such triggering position is so located as to commence the reversal operation for the first-mentioned compound piston as the control piston attains approximately the midpoint of its pressurizing stroke in one of the two directions of its linear reciprocation. In theory, this use of two double-acting pressure intensifiers with fluid control circuitry designed to operate the two pistons approximately 90.degree. out-of-phase with one another, would serve to avoid all of the previously mentioned disadvantages and to deliver hydrauic fluid to the output path free of any discontinuities. It should be noted, however, that throughout a major portion of the operation of such a system, both pressure intensifiers are in an active condition, delivering a relatively large quantity of hydraulic fluid into the output path. During the reversal of each double-acting pressure intensifier, the flow of hydraulic fluid into the output line will quickly drop off, approaching one-half of that level which was present prior to such reversal and which will again be present prior to the reversal of the other intensifier. Thus, surges in the rate at which hydraulic fluid will flow into the output line to the utilization device will be experienced. These surges can introduce substantial departures from uniformity in the operation of such a system.
Accordingly, improved methods and apparatus for overcoming some of the discontinuities and surge effects of such prior art methods and apparatus would clearly be advantageous.